Video disc players have inherent advantages in playing back video programs as compared with video tape players, e.g., the ability to quickly access the recorded information, relatively low replication costs, relatively simple mechanical requirements, etc. This, unfortunately, is accompanied by a basic disadvantage relating to tracking errors. Specifically, a forward or backward skip of one track on a helical scan VTR results in only a one field error in the location of the pick-up transducer. A single skip of one track convolution in a presently available video disc player (e.g., the RCA model SJT-100 results in an eight field error in the recovered signal. In another known player a skip of one track results in a two field error for disks recorded at a constant angular velocity.
It is known that one may correct tracking errors in a video disc player on a field-by-field basis, thereby placing video disc players on a par with video tape players. As an example, in U.S. Pat. No. 4,313,134 of Rustman, et al., a video disc system is disclosed wherein field numbers are recorded in pulse code modulation (PCM) form during the vertical interval of each video field. This has become known as a digital auxiliary information code (hereinafter DAXI code). Tracking errors (e.g., locked or skipped grooves or information tracks) are detected in a player by noting an improper sequence of two or more decoded DAXI field numbers. Correction of the error is provided by means of a transducer deflector (e.g., a stylus "kicker" solenoid or piezoelectric element) which radially translates the pick-up transducer in a direction to reduce the tracking error until such time as the field numbers again form an orderly sequence. Other patents relating to the DAXI code, its generation, recovery and use in video disc player include U.S. Pat. No. 4,308,557 of C. B. Dieterich, U.S. Pat. No. 4,307,418 of Mindel et al. and U.S. Pat. No. 4,309,721 of Christopher.
Heretofore it has been recognized that a need exists for correction of tracking errors in video disc systems on a more frequent basis than the field-by-field (inter-field) basis provided in known video disc systems. Such a system is described by C. B. Dieterich in his U.S. patent application Ser. No. 556,444 filed Nov. 30, 1983, entitled "VIDEO DISC ENCODING AND DECODING SYSTEM PROVIDING INTRA-FIELD TRACK ERROR CORRECTION".
In the Dieterich system, a method for forming a video disc recording signal, for facilitating intra-field track error correction, comprises providing a video signal and a data sequence representative of field numbers. The video signal, the data sequence and a subset of the data sequence are processed to form a recording signal with the subset data sequence conveying partial tracking information on a line-by-line or plural line basis within each field.
In a specific example of the Dieterich encoding system, the "partial" tracking data is conveyed by means of quadrature phase shift keying (QPST) of a subcarrier with local groove number (LGN) data and adding the QPSK signal to the mastering signal during a portion of the horizontal blanking interval for subsequent recovery in a video disc player. The player includes means for comparing an anticipated track number (from the recovered subsets) with the actual track number (from the corresponding portion of the full field number) to develop an error signal for radially translating the player pickup transducer in a sense to correct tracking errors when the numbers differ.